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A well-oxygenated eastern tropical Pacific during the warm Miocene

Nature volume 619, pages 521–525 (2023)Cite this article

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Abstract

The oxygen content of the oceans is susceptible to climate change and has declined in recent decades1, with the largest effect in oxygen-deficient zones (ODZs)2, that is, mid-depth ocean regions with oxygen concentrations <5 μmol kg−1 (ref. 3). Earth-system-model simulations of climate warming predict that ODZs will expand until at least 2100. The response on timescales of hundreds to thousands of years, however, remains uncertain3,4,5. Here we investigate changes in the response of ocean oxygenation during the warmer-than-present Miocene Climatic Optimum (MCO; 17.0–14.8 million years ago (Ma)). Our planktic foraminifera I/Ca and δ15N data, palaeoceanographic proxies sensitive to ODZ extent and intensity, indicate that dissolved-oxygen concentrations in the eastern tropical Pacific (ETP) exceeded 100 µmol kg−1 during the MCO. Paired Mg/Ca-derived temperature data suggest that an ODZ developed in response to an increased west-to-east temperature gradient and shoaling of the ETP thermocline. Our records align with model simulations of data from recent decades to centuries6,7, suggesting that weaker equatorial Pacific trade winds during warm periods may lead to decreased upwelling in the ETP, causing equatorial productivity and subsurface oxygen demand to be less concentrated in the east. These findings shed light on how warm-climate states such as during the MCO may affect ocean oxygenation. If the MCO is considered as a possible analogue for future warming, our findings seem to support models suggesting that the recent deoxygenation trend and expansion of the ETP ODZ may eventually reverse3,4.

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Fig. 1: Map showing the areal distribution and intensity of the ETP ODZ.
Fig. 2: Geochemical proxies relevant to Middle Miocene ETP oxygenation from Sites 845 and 872.
Fig. 3: Middle Miocene temperature.
Fig. 4: Hypothesis for the difference in conditions in eastern tropical Pacific between the MMCT and the preceding (warmer) MCO.

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Data availability

All data generated during this study are available as source data files for figures in which they appear and are available in the NOAA database at https://www.ncei.noaa.gov/access/paleo-search/study/37879. Source data are provided with this paper.

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Acknowledgements

We thank J. Wright and R. Mortlock for providing carbon and oxygen isotope analysis, K. Bu for help with trace-element analysis, B. Taphorn for his help with sample preparation for nitrogen isotope analysis and K. Wyman for help with scanning electron microscope images. We thank F. Boscolo-Galazzo and the two anonymous reviewers, whose comments improved the manuscript.

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Authors and Affiliations

  1. Department of Earth and Planetary Sciences, Rutgers, The State University of New Jersey, Piscataway, NJ, USA

    Anya V. Hess, Yair Rosenthal & Kenneth G. Miller

  2. Climate Geochemistry Department, Max Planck Institute for Chemistry, Mainz, Germany

    Alexandra Auderset & Alfredo Martínez-García

  3. Department of Geosciences, Princeton University, Princeton, NJ, USA

    Alexandra Auderset & Daniel M. Sigman

  4. Department of Marine and Coastal Sciences, Rutgers, The State University of New Jersey, New Brunswick, NJ, USA

    Yair Rosenthal

  5. State Key Laboratory of Marine Geology, Tongji University, Shanghai, China

    Xiaoli Zhou

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Contributions

A.V.H., Y.R., K.G.M. and A.A. conceived the study. A.V.H. and Y.R. composed the manuscript, with contributions from A.A., D.M.S. and A.M.-G. Geochemical analysis was done by A.V.H. (trace elements) and A.A. (nitrogen isotopes). Y.R. and X.Z. supervised the trace-elemental analysis and interpretation. A.M.-G. and D.M.S. supervised the nitrogen isotope analysis and interpretation. All authors reviewed the manuscript.

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Correspondence to Anya V. Hess.

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Extended data figures and tables

Extended Data Fig. 1 Age–depth correlation for Site 845 Hole A.

Size and shape of red boxes for carbon isotope correlation points indicate uncertainty. Oxygen and carbon isotopes are from C. mundulus sp. measured at this site. Ages and depths are from refs. 60,68,69.

Source data

Extended Data Fig. 2 Correlation of carbon isotope curves between Site U1338 and Site 845.

a, Site U1338. b, Site 845. Site U1338 data from Holbourn et al.34. Correlation points labelled 1–4; horizontal bars indicate uncertainty, which is also shown as box sizes in Extended Data Fig. 1.

Source data

Extended Data Fig. 3 Calibration of I/Ca and minimum water-column oxygen concentration.

Calibration of I/Ca in foraminifera and minimum water-column oxygen concentration, that is, oxygen concentration at depth of maximum depletion ([O2]min). Boxes indicate the ±1 s.d. range of I/Ca values for the MCO and the MMCT at Site 845 (Fig. 2a) and their interpreted [O2]min from this plot, in the case of the MCO, and from deductions in Hardisty et al.14 and nitrogen isotopes, in the case of the MMCT. Shading for [O2]min matches that in Fig. 1. Modified from Zhou, Hess et al.20.

Extended Data Fig. 4 Scanning electron microscope images of planktic foraminifera from Site 845.

Images show outside, inside and cross-sectional views. Note similar quality of preservation (moderate to poor) throughout the study interval. Scale bars are 50 μm.

Extended Data Fig. 5 Map showing location of sites used in regional SST compilation.

Base map is modern SST from the NOAA70. Site 845 palaeolocations from Shipboard Scientific Party60, other palaeolocations from ODSN Plate Tectonic Reconstruction Service.

Extended Data Fig. 6 SST by region.

Data used to construct Fig. 3a. SST by region relative to their averages from 16–15 Ma, smoothed using 100-kyr bins. Inverted triangles indicate age control points, coloured by site68,69,71,72. To compare Mg/Ca data from different foraminifera species, some without modern equivalents and therefore lacking species-specific temperature calibrations, we calculate temperatures using the same multispecies equation at all sites (see Methods). Temperatures derived from TEX86 data are only available for one site and so are presented using the original authors’ calibrations45. In constructing Fig. 3a, for the site with TEX86 data, those temperatures are used rather than Mg/Ca-derived temperatures and in this figure, Mg/Ca-derived temperatures are dashed. a, West Pacific warm pool42,43,44. b, ETP50. c, Southern Ocean45,73.

Source data

Source data

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Hess, A.V., Auderset, A., Rosenthal, Y. et al. A well-oxygenated eastern tropical Pacific during the warm Miocene. Nature 619, 521–525 (2023). https://doi.org/10.1038/s41586-023-06104-6

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